The present invention generally relates to new and improved polyamic acid/imide microlithographic compositions, their method of manufacture, and particularly their use in a novel concurrent wet-development and improved lift-off process.
Photoresist compositions are commonly used in microlithographic processing and generally consist of a diazoquinone photosensitizer and a novolak resin binder. Normally, such compositions are coated onto semi-conductor substrates; and, when exposed to light of the proper wavelength, they are chemically altered in their solubility to alkaline developer solutions. Positive-working photoresists are initially insoluble in the alkaline developer, but after exposure to light, the exposed regions will dissolve or "wet-develop" in alkaline solution forming indented, micron-size line features. Subsequently, for many applications, the undissolved portion of the resist must be stripped from the substrate.
Positive-working novolak photoresists, however, are being increasingly used under conditions which render them insoluble in conventional strippers. Ion implantation, plasma hardening, deep UV hardening and other high temperature processing conditions produce, for example, crosslinking reactions within the resist. This makes stripper penetration and resist dissolution, which are essential to removal of the resists, virtually impossible.
Oxidative strippers such as hot sulfuric acid-hydrogen peroxide mixtures can be effective against intractable resists, but removal is often slow or incomplete. Moreover, these treatments are restricted to use on unmetallized substrates. Alternatively, removal of intractable resists is sometimes possible by soaking in hot chlorinated and/or phenolic solvents. However, toxicity and disposal problems associated with these materials are critical drawbacks to their use.
In the past there have been attempts to remove, otherwise intractable, photoresist compositions from metallized substrates with safe stripper solvents, devoid of the prior art problems. For example, IBM's U.S. Pat. No. 3,873,361 taught that novolak photoresist hardbaked at 210 degrees centigrade could be stripped with the conventional stripper solvent N-methyl pyrrollidone. Although this liftoff process was specifically designed to accommodate the formation and removal of metallic masking layers above the resist, in practice, it simply failed to work because the N-methyl pyrrollidone did not dissolve the hardened photoresist.
H. A. Protschka (IBM), European Patent Application 0 257 255 (1987).
One such solvent-soluble release layer material is polysulfone. This material has the advantage of enabling the liftoff of metal mask layer by conventional solvent stripping. Although the material also serves to insulate and protect the metal substrate from attack by harsh oxidative strippers, it all the same requires such harsh strippers to liftoff the photoresist layer if hardened. Polysulfone can serve to liftoff the photoresist material itself as in the fifth step of FIG. 3, but not without other drawbacks.
Such polysulfone release layers are insoluble to conventional alkaline developing solutions. Accordingly, unlike the photoresist they are not "wet-developable". Patterns in the release layer must be made by "dry-development" with, for example, reactive ion etching. In fact, little if any commercial use has been made of these special release layers, in large part because they must be dry-developed.
In conventional microlithography, micron feature sizes developed by dry etching are excessively more expensive than wet etching. Additionally, the dry developable release layer material requires separate plasma etching equipment in addition to that required to etch the photoresist layer and other layers of multilayer microlithographic processing. The addition of even more equipment and more steps becomes such a serious drawback that those in the art have preferred the toxicity, disposal, and other restrictions associated with employing non-conventional strippers, to remove the photoresist layers, rather than deal with a dry-developable release layers. Some otherwise acceptable release layer materials do not adhere sufficiently to semi-conductor substrates or are incompatible surfaces for applying resist layers, or other organic or inorganic layers, thereto.
Accordingly, a wet-developable release layer of material that could be co-developed concurrently with the photoresist material, without requiring separate plasma etching equipment, and which could be lifted off by immersion in more mild and less toxic solvents and which would not erode metallized substrates, while providing good adhesion to semiconductor substrates and a compatable surface for applying resist layers, or other organic or inorganic layers, would be a surprising advancement in the art fulfilling a long felt need in the industry.